Look into LOCK-N-STITCH Link
But they're pretty proud of their system, and the location of the crack may make it more difficult.
If you were to keep the coolant pressure super-low, an epoxy repair with something like Devcon Plastic Steel might work.
Welding would require stripping the block down, preheating, welding, then SLOWLY cooling it off. But even then, you might get warping.

 

Regarding overall engine health, I forgot to mention that I found shavings in the oil, probably more than I’ve ever seen come out of an oil pan. Reportedly it hadn’t been regularly driven for 10 years, no record of the last oil change. Fresh oil & filter quieted down the valvetrain and brought the oil pressure gauge back to life (I assumed I had a bad sender).

Do these engines have a tendency to produce more metal than other designs? Given the number of miles and light duty use reported by the PO, is there a likely source of the metal?

Bottom line, even though the engine seems perfectly healthy and even if the shavings aren’t a sign of impending failure, there’s no question that it needs to be rebuilt. Other than finding a solution for the block, I think my biggest question is if I would be gaining anything (from the standpoint of performance as well as practicality) from rebuilding the heads.

 

A few days ago I spoke with the owner of a small local machine shop and sent him a few photos, he said he needed to make a few phone calls but would get back to me. He called yesterday and it doesn’t sound like anyone he spoke with has ever dealt with a similar problem. He did, however, look up the engine specifications and seemed to have a good understanding of what caused the issue. To summarize his opinion on the matter, the side of the blocked cracked because it’s too thin, therefore it’s too thin to repair; even without the extreme uneven heating caused by the block heater, the stress caused by normal heating and cooling cycles would, maybe after one year or maybe after 20, cause the repair to fail.

Based on previous threads and my conversation with the machine shop (I also personally have some work experience with non-automotive metallurgy and related topics), my conclusion is that it’s not worth it to try to repair the block, nor is it worth it to try to put my heads or internals on another block.

Used 6.9 and 7.3 IDI engines aren’t littering junkyards across the US anymore, and at least in my area (where they all rusted out) you definitely can’t go out and buy a good running truck for a couple grand. For the sake of simplicity, going with another 6.9 sounds like my best bet, and I was already able to find two used engines within an hour of home. One reportedly came out of a low mile truck, I guess we’ll see.

 

...extreme uneven heating caused by the block heater...

It doesn't cause extreme anything. It never gets above 250°F, which isn't enough to warp an engine, even if it's near freezing. The leak being near the heater is just coincidence - if you pressure-wash the whole block, you might find other cracks far from the heater.

 

Sounds like you've done your homework. With the crack in the block and metal shavings in the oil, it sounds like your engine is done for. Finding a decent used replacement is like a pig in a poke. Hopefully that low mileage truck engine pans out. Good luck!

 

Fingers crossed. I’ve never had to do a full rebuild or replace an engine, nothing I currently own has an engine or transmission that wasn’t there from the factory. 15 years ago, when I was buying and selling 12 cars, trucks and motorcycles every year I made some questionable used axle and transmission swaps. 15 years ago I probably would have put this thing right back up for sale, but this is something I’ve always wanted and because I’ll probably own it forever, I think it’ll be worth the hassle.

 

...asymmetric was what I actually meant.

That's even less-significant. If an engine block (especially a diesel) could be damaged by uneven temperature (especially as LITTLE unevenness as a small, slow, low-temperature block heater is capable of causing), it wouldn't make it past the first prototype casting. It would shatter just cooling down from being cast.

Are you the same Steve83 from FSB?

Yes

...your information, pictures and diagrams...have been particularly invaluable on that project, thank you.

Glad to hear someone is still getting some use from them.

 

Well, I’ve probably got another $1500 and maybe 6 hours worth of wasted time left in me before this truck gets parked down by the creek with an ‘86 F150 and a ‘79 Mercedes 300D where it will experience a slow but steady death from getting parted out.

I picked up that used 6.9 from an ‘86 that had been sitting in a shed since it was wrecked in ‘91. $800 and most of a day to get it. Today I got it mounted on an engine stand (I of course had to buy a new one because of course an IDI isn’t going to fit on a 1000-pound stand) and got the heads and oil pan pulled.

I now know (these are questions that in hindsight should have been asked) that the engine did not run after the wreck. In fact, the wreck was so bad that the front clip was pushed into the fan and that the C6 couldn’t be salvaged because it was in pieces. I know this because I called the guy I bought it from after discovering a bent rod, with a corresponding bent valve, with a corresponding piston that had mostly turned to shrapnel. I’m not an engine guy but even if the flywheel was stopped dead in its tracks I can’t quite figure out how that could cause this. And while I have relatively limited experience doing much more than a head gasket, the wear on the cylinder walls, cam wear, sludge and rebuilt IP make me pretty skeptical of the whole thing.

I saw a used T444E on marketplace that I’m going to look at, since my $20k budget is long gone and I’m going to have to spend some cash anyway, I figured I’d explore that rabbit hole. (No, I’m not a fan of blasphemous frankentrucks but my first diesel truck was a 2003 7.3 and the first thing I ever drove was a Bricknose F250 so I’m giving myself permission out of sentimentality.)